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Data from NASA's Wide-field Infrared Survey Explorer (WISE) has uncovered what has been hailed as the coolest class of stars to be found, with temperatures dipping as low as that of the human body.

Using visible-light telescopes to hunt for these dark stars termed Y dwarfs, astronomers have been unsuccessful for more than a decade due to their cool temperatures making them nigh on impossible to detect. It is only now, with the help of WISE's infrared vision, that scientists have been able to locate the faint glow of six Y-dwarfs which lie at a distance of around 40 light-years, sitting in relatively close proximity to our Sun.

This artist's conception illustrates what a "Y dwarf" might look like. Y dwarfs are the coldest star-like bodies known, with temperatures that can be even cooler than the human body. Image: NASA/JPL-Caltech.

“WISE scanned the entire sky for these and other objects, and was able to spot their feeble light with its highly sensitive infrared vision,” says Jon Morse, Astrophysics Division director at NASA headquarters in Washington. “They are 5,000 times brighter in the longer infrared wavelengths WISE observed from space than those observable from the ground.”

Due to their low temperatures, these stars are cool customers, belonging to a family of failed stars also known as brown dwarfs. Being so low in mass means that they are unable to fuse atoms in their cores and so they do not violently burn like their stellar cousins which shine steadily for billions of years. Y dwarfs prefer to take it easy, cooling and fading with time, emitting the little light that they have left at infrared wavelengths.

Of the 100 brown dwarfs sniffed out so far by WISE, six are classified as cool Y-type stars, with the coolest, WISE 1828+2650 having an atmospheric temperature of less than 25 degrees Celsius – about as cool as the room that you are sitting in now.

“The brown dwarfs we were turning up before this discovery were more like the temperature of your oven,” says WISE science team member, Davy Kirkpatrick, of the Infrared Processing and Analysis Center at the California Insitute of Technology and is lead author of a paper appearing in the Astrophysical Journal Supplement Series, which describes the 100 confirmed brown dwarfs. “With the discovery of Y dwarfs, we've moved out of the kitchen and into the cooler parts of the house.”

In order to confirm their finding, the WISE team narrowed down their brown dwarf list with the help of NASA's Spitzer Space Telescope and commandeered powerful instruments, such as the ground based telescope at the Keck Observatory in Hawaii, to split incoming light in their search for molecular signatures of water, methane and possibly ammonia. The new and cooler Y-dwarfs were handled by NASA's Hubble Space Telescope where they were identified based on changes in their spectral features compared to common brown dwarfs, indicating a low atmospheric temperature.

“Finding brown dwarfs near our Sun is like discovering there's a hidden house on your block that you didn't know about,” says Michael Cushing, a Wise team member at NASA's Jet Propulsion Laboratory and who is lead author of second paper that details the Y dwarfs in the Astrophysical Journal . “It's thrilling to me to know we've got neighbours out there yet to be discovered. With WISE, we may even find a brown dwarf closer to us than our closest known star [Proxima Centauri, at a distance of four light-years away].”

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